Project Summary/Abstract Huntington disease (HD) is caused by a mutation in exon 1 of the gene for Huntingtin (HTT). The mutant protein (mHTT), which contains an expanded stretch of glutamine residues (polyQ), is responsible for all HD manifestations, including dysfunction and degeneration of neurons. The overall goal of this Project is the discovery and development of effective treatments for HD. The Project is informed by evidence that mHTT is a client protein of the cytosolic chaperonin TRiC and that TRiC overexpression prevents or rescues mHTT- induced phenotypes in model systems. In particular, we have shown that individual TRiC subunits (CCTs), as well as the soluble apical portion of CCT1 (ApiCCT1), reverse deficits in BDNF trafficking and signaling in BACHD neurons in vitro. Project 2 will decipher whether or not TRiC and TRiC-derived proteins (hereafter, TRiC reagents) act in vitro to prevent dysfunction and degeneration of cortical and striatal neurons from the BACHD model. Our hypothesis is that that increasing the levels of TRiC or TRiC-derived reagents will abrogate and/or reverse mHTT- linked pathogenesis. We propose four Aims to examine in vitro quantitative and temporal features of HD pathogenesis. In Aim 1 we will further define deficits in axonal trafficking and signaling of BDNF in the BACHD model in vitro; we will confirm and further explore the ability of TRiC-derived reagents to prevent or reverse deficits. We will begin with reagents proven effective (CCT3, 5 and Apical CCT1) and test reagents coming from Project 1. In Aim 2, we will define deficits in BACHD neuron phenotypes: synapse formation, synaptic connectivity, gene expression, structure of neuronal somas and processes, mitochondrial function and calcium homeostasis. The timing of and possible progression of deficits will be defined and the impact of TRiC reagents explored. In Aim 3, we will define deficits in proteostasis in BACHD neurons examining clearance of mHTT that may involve protein folding, ER stress, and the UPS and autophagy/lysosome pathways. Whether introduction of TRiC reagents prevent and/or reverse changes will be examined. In Aim 4, we will examine iPSC-differentiated neurons from HD patients to determine whether deficits defined in Aim1-3 are detected and, if so, determine whether they are TRiC reagent-responsive. Project 2 is expected to enhance considerably understanding of HD pathogenesis and to define a novel approach to potential treatment. It is possible that TRiC and TRiC-inspired approaches may elucidate pathogenesis and treatment of other neurodegenerative disorders, including Alzheimer disease, Parkinson disease and ALS.